Fermentation control

The aims of fermentation control may be summarised as:

(1) production of beer of consistent flavour and analytical composition;

(2) maximisation of rates of conversion of sugar to alcohol, within the constraints of maintaining high product quality;

(3) ensuring that vessel residence times are consistent and as short as possible, both to maximise fermenter productivity and to allow brewing to be performed to a predictable time-table.

In the majority of fermentations, control is exerted through regulation of initial conditions and thereafter via attemperation. The methods used to control the initial parameters, namely, wort composition, temperature, yeast pitching rate and wort dissolved oxygen concentration, are discussed in Section 6.1. In practice, wort composition and the total extract delivered to the fermenter are controlled up-stream of the fermenter. Therefore, the variables available for manipulation are restricted to temperature, pitching rate, pressure and dissolved oxygen concentration. The first requirement of fermentation control is to choose appropriate values for these para meters and put systems in place to ensure that the desired values are achieved. In the vast majority of breweries, this - that may be described as a 'passive' system - is the extent of fermentation control. A second possibility is to use an interactive control strategy. This recognises the probability that there will be some variability that a passive control system is unable to correct. In particular, variations in wort composition and yeast physiological condition are likely.

Three levels of sophistication of active control strategy are possible. First, the use of preliminary treatments to remove sources of inconsistency. This approach obviates the necessity to modulate the values of key user-variables. For example, variability in the physiological condition of pitching yeast requires compensation by selecting an appropriate pitching rate and/or wort dissolved oxygen concentration. A pre-treat-ment which eliminates the physiological variability and produces pitching yeast of consistent condition would allow fixed values of pitching rate and dissolved oxygen concentration to be used. A treatment such as this would be oxygenation of yeast before pitching to promote sterol synthesis (see Section 6.4.2.2).

The second option is to make off-line assessments immediately prior to pitching which allow appropriate values for user variables to be chosen for that particular fermentation. For example, the so-called 'yeast vitality' tests whose aim is to produce a rapid assessment of yeast physiological condition, and thereby allow an optimum pitching rate and/or wort oxygenation regime to be chosen (see Section 7.4.2). An automated example of this type of approach is to measure the specific rate of oxygen uptake, by yeast, during wort collection. From this, the optimum oxygen concentration for that particular fermentation may be computed and added to the inflowing wort (see Section 6.3.6).

The third option is the use of interactive automatic control systems. This requires one or more parameters to be monitored, in real-time. In response, automatic adjustments are made to appropriate control elements. In theory, this ensures that the fermentation proceeds according to a desired profile. Parameters that may be measured on line and that produce outputs suitable for use in feedback loop systems are discussed in Section 6.3. They include exothermy and profiles of C02 evolution and ethanol production. The use of these and other parameters in automatic control systems in brewery fermentations is discussed more fully in Section 6.4.2.

6.4.1 Effect of process variables on fermentation performance

Interactions between yeast pitching rate, temperature and dissolved oxygen concentration in fermentation are complex. They influence fermentation efficiency, measured both as vessel residence time and the balance between ethanol yield and yeast growth. In addition, although wort composition and the yeast strain are important determinants of beer flavour, they influence the formation of flavour-active metabolites. Choice of appropriate values for temperature, pressure, pitching rate and wort oxygen concentration involves some compromise. Thus, inevitably there will be a play-off between fermentation efficiency and production of beer with a balanced spectrum of flavour components. Although the combined effects of process variables on fermentation performance are complex, it is possible to make some generalisations regarding their individual contributions. These are discussed in this section. An attempt has been made to distinguish between effects on fermentation efficiency and those that influence product quality. Where possible, the effects of simultaneously varying more than one parameter are described.

6.4.1.1 Temperature. Temperature has an effect primarily on fermentation rate through its effect on yeast growth and metabolic rate. The data in Fig. 6.18 shows attenuation profiles for pilot-scale high-gravity lager fermentations performed at a range of temperatures between 11°C and 25°C. All other process variables were, as near possible, identical. As may be seen, between the two extremes of temperature, there was a difference of nearly 100 hours in the time taken to achieve final gravity. Based on this result it may be supposed that it must be advantageous to perform fermentations at as high a temperature as possible. Most brewing yeast strains have a maximum growth temperature within the range of 30 to 35°C, suggesting that very rapid fermentations could be achieved. In fact, several factors preclude the use of very high temperatures. Some loss of volatile flavour components and ethanol by gas stripping is inevitable in all fermentations. The severity of this effect is obviously correlated with the vigour of fermentation. At very high temperatures, the rate of loss becomes unacceptable. Indeed, at 30°C, it would be necessary to control the loss of water due to evaporation and it would be difficult to retain the fermenting wort within the vessel!

Fig. 6.18 Pilot scale (800 hi) lager fermentations performed at the temperatures shown (°C). In all fermentations the pitching rate was 12 x 10s cells ml-1 and the initial wort dissolved oxygen concentration was 25 ppm (Boulton & Box. unpublished data).

Of greater significance is the effect of temperature on the formation of yeast-derived flavour components. In general, ales are fermented at higher temperatures (typically 18 to 25°C) compared to lagers (6 to 15°C). Ales tend to be more highly flavoured than lagers, whereas lagers contain greater concentrations of more subtle flavour components. The greater flavour intensity of ales is in part due to the malts and hops used in their preparation. In addition, the beers also usually contain greater concentrations of higher alcohols and esters, which are produced during fermentation.

Some of these differences are a consequence of the yeast strains used. Generally, ale strains produce more esters and higher alcohols than lager strains. However, increased temperature also produces elevated levels of higher alcohols and possibly esters (Stevens, 1960; Mandl et al., 1975; Kumada et al., 1975; Posada et al, 1977; Miedener, 1978). It is probable that it is only in the case of higher alcohols that the increase can be ascribed directly to increase in temperature. In the case of esters, effects due to oxygen may also be implicated. Thus, elevation in temperature reduces the solubility of oxygen. There is a negative correlation between oxygen availability and ester synthesis. Decreased oxygen availability reduces yeast growth extent because of sterol limitation. Reduced utilisation of wort amino nitrogen and sugars for biomass formation allows a greater proportion of the pools of oxo-acids and acetyl-CoA to be used for the synthesis of esters. In fact, it may be shown that yeast growth extent and beer ester concentrations are unchanged at different temperatures when oxygen concentration is maintained at a constant value.

The temperature at which fermentation is conducted may influence oxygen requirement. As the temperature at which yeast is grown is reduced, the cells incorporate a progressively greater proportion of unsaturated fatty acid residues into the plasma membrane in order to maintain fluidity (Boulton & Ratledge, 1985). It follows that de novo unsaturated fatty acids synthesis during fermentation will require more oxygen at lower fermentation temperatures.

Fermentation temperature influences the profile of formation and reduction of VDK. Thus, increased fermentation temperature increases the peak value of VDK. In addition, at elevated temperatures the VDK peak occurs earlier in the fermentation and the rates of formation and decline are more rapid (Fig. 6.19). Thus, elevated temperature favours rapid primary and secondary fermentation.

The choice of temperature has practical implications for fermentation management. The use of high temperatures results in increased fermentation rate but some of the potential advantage is lost because of the concomitant increase in time taken to

Fig. 6.19 Profile of vicinal diketone formation and reduction during high-gravity (1060) lager fermentations performed at the temperatures shown (°C). In all cases pitching rates were 15 x 106 cells ml"1 and the initial oxygen concentration was 25 ppm. Vicinal diketones were determined by gas chromatography (EBC Analytica, 1987) (Boulton & Box, unpublished data).

Fig. 6.19 Profile of vicinal diketone formation and reduction during high-gravity (1060) lager fermentations performed at the temperatures shown (°C). In all cases pitching rates were 15 x 106 cells ml"1 and the initial oxygen concentration was 25 ppm. Vicinal diketones were determined by gas chromatography (EBC Analytica, 1987) (Boulton & Box, unpublished data).

cool at the end of fermentation. Vigorous fermentations require more careful handling with respect to control of fobbing. Care must be taken to protect yeast in large closed vessels when fermentations are performed at high temperatures. In traditional top-cropping ale fermentations performed in open square vessels, the yeast is removed comparatively early in the process. Consequently, it is not subject to heat stress. Conversely, similar fermentations performed in large-capacity closed vessels provide an environment in which it is much more difficult to control the temperature of the yeast. Thus, when yeast cells are packed into the cone of a vessel, attemperation is inefficient and difficult. Clearly, this problem will be exacerbated where a high fermentation temperature is used. In such cases, the yeast should be removed from the vessel as soon as is possible and vessels should be fitted with cone cooling jackets (see Section 5.4.1).

6.4.1.2 Yeast pitching rate. Fermentations are pitched within the range of 5 x 106 cells ml 1 for a sales-gravity ale up to 20 x 106 cells ml 1 for a high-gravity lager. In weight terms these rates equate to c. 1.5g 1 1to6.0gl 1 wet weight (c. 0.3-1.2gl 1 dry weight). The pitching rate must be based on the gravity of the wort and should produce a fermentation that is as rapid as possible without compromising beer quality or the size of the yeast crop.

The pitching rate influences both fermentation rate and the extent of yeast growth. Predictably, within certain limits, there is a positive correlation between yeast pitching rate and fermentation rate. At low values, there is also a positive correlation between pitching rate and the extent of new yeast growth during fermentation. However, at progressively higher pitching rates new yeast growth reaches a peak and thereafter declines (Fig. 6.20). The pattern of yeast growth may be explained in terms of the ratio of the initial yeast cell concentration and the concentration of available nutrients. In most microbial batch cultures, the number of cells in the inoculum is small compared to the terminal cell concentration when growth is limited by exhaustion of a nutrient.

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